1. Field of the Invention
Embodiments of the present invention generally relate to testing systems for large area substrates having electronic devices formed thereon and, more particularly, to locating driver defects for these electronic devices.
2. Description of the Related Art
Active matrix liquid crystal displays (LCDs) are commonly used for applications such as computer and television monitors, cell phone displays, personal digital assistants (PDAs), and an increasing number of other devices. Generally, an active matrix LCD comprises two flat plates or panels having a layer of liquid crystal materials sandwiched therebetween. The flat plates are typically made of glass, a polymer, or other material suitable for having electronic devices formed thereon. One of the flat plates typically includes a conductive film disposed thereon and may be referred to as a color filter. The other flat plate typically includes an array of thin film transistors (TFTs), each coupled to a pixel. Each pixel is activated by providing signals to driver circuits, such as data and gate lines, and activation of the pixel may be provided by simultaneously addressing an appropriate data line and gate line. Each TFT may be switched on or off to generate an electrical field between a TFT and a portion of the color filter. The electrical field changes the orientation of the liquid crystal material, creating a pattern on the LCD.
Because of the high pixel densities, the close proximity of the gate lines and data lines, and the complexity of forming the TFTs, there is a high probability of defects during the manufacturing process. Known testing methods for high density LCD panels include contact testing methodologies, which require connection to and testing of each individual row/column intersection within the panel array. For such testing, advanced probing technology is necessary to establish reliable contacts among the densely populated pixel elements. A high density LCD array panel typically includes 640 by 480 pixels, and a typical test time for such a panel is approximately 2 hours. For a color filter having the three primary colors (i.e., red, green, and blue), a typical test cycle requires additional connections and additional testing time. The time and expense of testing, although necessary, may often be a limiting factor to the commercial success of large array LCD panels.
Prior art methods of detecting defects in the LCD panel having the array of TFTs are limited to isolation of a particular area of the TFT array without ascertaining the precise location of the defect. Additionally, an inoperable pixel may be identified, and the cause of the failure of the pixel may not be ascertained. For example, the identified pixel itself may not be defective, but may not be operational due to other factors, such as a faulty driver circuit. In this case the identified pixel may otherwise be in operable condition, but for the driver circuit defect. When this defective driver circuit is identified and localized accurately, the defect may be analyzed and repaired, which facilitates operation of the identified pixel.
One known method to identify driver defects is to recognize a certain geometrical pattern of pixel defects and determine with evaluation software that this pattern is caused by a driver defect. This determination includes localization of the driver defect by reporting the line coordinates of the pattern, or the coordinates where the pattern starts or ends. For example, if all or many pixels along a line have a defect and the density of the defective pixels does not change from one side to the opposite side of the display, this may be ascertained to be a driver defect. The location would be the location of this line. In another example, many pixels of many lines are failing starting at a certain line number n. All lines >n have these failing pixels, while all lines <n do not have failing pixels. The evaluation software could identify this geometrical pattern of pixel defects as a driver defect, and the location would be the line n where the pattern starts. However, there is a risk that this geometrical evaluation may incorrectly report a driver defect even if the problem is a line defect or a coincidental grouping of pixel defects.
Therefore, a need exists in the art for a faster and more accurate testing method in an effort to reduce the production costs of LCD panels.